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Former Russian double agent Sergei Skripal and his daughter Yulia, who were targeted in last Sunday's attack, remain in a critical but stable condition in hospital, she said.

"It is still very serious for the two people who were indeed the subject of this outrageous attack," Ms Rudd said, adding Britain needed to determine the source of the nerve agent.

"In terms of further options, that will have to wait until we're absolutely clear what the consequences could be, and what the actual source of this nerve agent has been."

The home secretary was speaking after chairing an emergency meeting to discuss the poisoning as police, backed by soldiers, continued to search the town where the Skripals were attacked.

She said that the process of collecting evidence had to continue to ensure the attribution for the attack was clear, and the investigation had been painstaking and involved more than 250 counter-terrorism officers.

The meeting was similar to the ones convened after extremist attacks and other threats to national security.

It covered the latest police and intelligence reports from Salisbury, where the military-supported investigation has turned to the cemetery where the ex-spy's wife and son are buried.

Police are looking for clues as to what sickened Mr Skripal, 66, who in 2006 was convicted in Russia of spying for Britain, and his daughter, Yulia, 33.

The father and daughter were found unconscious on a bench in Salisbury, a town 140 kilometres southwest of London.

A local restaurant and pub have been searched and remain closed to the public.

Anti-contamination measures have been taken at places the father and daughter visited before they fell ill.

Police in hazardous material suits have also collected evidence from Mr Skripal's house.

About 180 troops, including some with chemical expertise, have been sent to the city to remove ambulances, other vehicles involved in the incident and other possibly contaminated objects.

"The public should not be alarmed," counter-terrorism police, who are leading the investigation, said in a statement.

British Prime Minister Theresa May said her Government would respond appropriately if evidence showed Moscow was behind the attack on Mr Skripal, who served time in a Russian jail for spying for England, before he was released in a spy swap.

The Kremlin has denied any involvement in the incident and said anti-Russian hysteria was being whipped up by the British media.